Malorientation in half-bivalents at anaphase: analysis of autosomal laggards in untreated, cold-treated, and cold-recovering crane fly spermatocytes.

Exposing crane fly larvae to 6 degrees C or returning them to 22 degrees C after exposure to 6, 2, or 0.2 degrees C can induce any number of autosomes in their primary spermatocytes to lag near the spindle equator at anaphase. Autosomal laggards in cold-recovering cells are contained in bivalents until anaphase (Janicke, M. A., and J. R. LaFountain, 1982, Chromosoma, 85:619-631). We report here documentation that lagging autosomes in cold-treated and cold-recovering cells are maloriented. During meiosis I, half-bivalents usually associate with only one pole via kinetochore fibers, with sister chromatids being oriented to the same pole. In contrast, laggards had kinetochore microtubules (kMTs) extending from them toward both poles: one sister was oriented to one pole and the other had some or all of its kMTs extending toward the opposite pole. Bipolar malorientation of autosomal laggards also was observed in one untreated cell. The number of kMTs per half-bivalent was similar in lagging and non-lagging autosomes, and those kMTs were contained in long birefringent kinetochore fibers. The overall spindle structure in cold-recovering cells was similar to that observed in untreated anaphase cells. Giemsa-stained centromeric dots of sister chromatids were contiguous in non-laggards and separated in laggards at anaphase. We conclude that bipolar malorientations can exist at anaphase in chromosomes that remain paired until anaphase, that cold recovery increases the frequency of that anomaly, and that such malorientations may be one cause of anaphase lag.